Vehicles often include both front and rear steerable wheels and a plurality of steering modes. The steering modes include, for example, modes in which the vehicle is steerable with only the front wheels, only the rear wheels, the front and rear wheels being steered in opposite directions, or the front and rear wheels being steered in the same direction. When the vehicle changes from one steering mode to another mode while it is in the process of turning, it is advantageous to prevent the vehicle's rate of turn from significantly deviating from the operator's desired rate of turn. As can be appreciated, if the rate of turn of the vehicle is sharply changed without any change in the operator's steering input, the vehicle can become unstable or can be steered in an unintended direction.
Early controls for vehicles having all-wheel steering, such as that disclosed in U.S. Pat. No. 3,279,813, issued to Linsay on Oct. 18, 1966, were hydromechanical and did not provide any means for maintaining the rate of turn of the vehicle when the operator changes from one steering mode to the next. Hence, each position of the steering wheel is associated with a plurality of rates of turn corresponding to each respective steering mode. For example, if the operator actuates the selector box to change from the front wheel steering mode to the cramp steering mode (front and rear wheels being steered in opposite directions), the front wheels are maintained in the direction they had previously been directed and the rear wheels are automatically turned in the opposite direction to an angle corresponding to the angle of turn of the front wheels. Therefore, as the rear wheels are turning, the vehicle's rate of turn is significantly increased even though the operator is not moving the steering wheel. Likewise, when the operator switches back to the front steering mode, the rear wheels will automatically return to a straight-ahead or neutral position. As the rear wheels straighten, the vehicle's rate of turn is significantly decreased even though the operator has not changed the position of the steering wheel. As noted above, these resulting changes in the vehicle's rate of turn can cause handling problems. This is particularly true at relatively high speeds; however, even at low speeds, such changes can be very disconcerting to the operator.
An alternative system used in automobiles is disclosed in U.S. Pat. No. 4,621,702, issued Nov. 11, 1986 to Kanazawa et al. While this system prevents the above mentioned control problems, the vehicle only changes steering modes while the position of the steering wheel is being changed. Therefore, if the steering wheel is maintained in a given position, the steering angle of the wheels is not changed. While this allows the operator to maintain control of the vehicle, the steering mode can only be changed in response to changes in the position of the steering wheel. For example, if the steering wheel is only turned a small amount, the vehicle will be in the crab steering mode (front and rear wheels are turned in the same direction). Likewise, if the steering wheel is turned a large amount, the vehicle will be in the cramp steering mode. When the steering wheel is in an intermediate position, the vehicle will be in an intermediate steering mode between the cramp mode and the crab mode. It is therefore impossible to place the vehicle in any steering mode other than those that are predetermined by the vehicle designer for a given amount of steering input.
The latter steering control is not suitable for use on construction vehicles, such as wheel type loaders, tractors, or trucks. Such a vehicle must allow the operator to change the steering mode in response to the application in which the vehicle is being used. The vehicle must be operable in a particular steering mode irrespective of the operator's steering input. To improve productivity, the vehicle should be capable of changing steering modes while the vehicle is operating. As noted in the discussion of the former steering control, however, such transitions should not cause the vehicle's rate of turn to significantly differ from the operator's steering input so that the vehicle does not become unstable.
The present invention is directed at overcoming one or more of the problems as set forth above.